Carbon nanotube supported platinum-palladium nanoparticles for formic acid oxidation

Pt, Pd and Pt_xPd_y alloy nanoparticles (Pt₁Pd₁, Pt₁Pd₃, atomic ratio of Pt to Pd is 1:1, 1:3, respectively) supported on carbon nanotube (CNT) with high and uniform dispersion were prepared by a modified ethylene glycol method. Transmission electron microscopy images show that small Pt and Pt_xPd_y nanoparticles are homogeneously dispersed on the outer walls of CNT, while Pd nanoparticles have some aggregations and comparatively larger particle size. The average particle sizes of Pt/CNT, Pt₁Pd₁/CNT, Pt₁Pd₃/CNT and Pd/CNT obtained from the Pt/Pd (2 2 0) diffraction peaks in the X-ray diffraction patterns are 2.0, 2.4, 3.1 and 5.4 nm, respectively. With increasing Pd amount of the catalysts, the mass activity of formic acid oxidation reaction (FAOR) on the CNT supported catalysts increases in both cyclic voltammetry (CV) and chronoamperometry (CA) tests, although the particle size gets larger (thus, the relative surface area gets smaller). The CV study indicates a ‘direct oxidation pathway’ of FAOR occurred on the Pd surface, while on the Pt surface, the FAOR goes through ‘CO_ads intermediate pathway’. Pd/CNT demonstrates 7 times better FAOR mass activity than Pt/CNT (2.3 mA/mgPd vs. 0.33 mA/mgPt) at an applied potential of 0.27 V (vs. RHE) in the CA test.     

Size-dependent antimicrobial properties of sugar-encapsulated gold nanoparticles synthesized by a green method

The antimicrobial properties of dextrose-encapsulated gold nanoparticles (dGNPs) with average diameters of 25, 60, and 120 nm (± 5) and synthesized by green chemistry principles were investigated against both Gram-negative and Gram-positive bacteria. Studies were performed involving the effect of dGNPs on the growth, morphology, and ultrastructural properties of bacteria. dGNPs were found to have significant dose-dependent antibacterial activity which was also proportional to their size. Experiments revealed the dGNPs to be bacteriostatic as well as bactericidal. The dGNPs exhibited their bactericidal action by disrupting the bacterial cell membrane which leads to the leakage of cytoplasmic content. The overall outcome of this study suggests that green-synthesized dGNPs hold promise as a potent antibacterial agent against a wide range of disease-causing bacteria by preventing and controlling possible infections or diseases.     

Description of melting of aluminum nanoparticles

A new physicomathematical model is proposed for describing the process of melting of aluminum nanoparticles, which takes into account the dependences of thermophysical variables on the temperature and particle size obtained by the molecular dynamics method. The study is performed for samples with spherical, cylindrical, and plane symmetry. The times of melting of aluminum nanoparticles are found as functions of the nanoparticle radius and ambient temperature. Two-front melting modes are observed for the first time; these modes are the result of the scale factor in the dependence of the melting temperature on the particle size.     

On the melting temperature of nanoparticles

A theoretical explanation is offered for the possibility of melting a nanoparticle at a higher temperature as compared to the melting temperature of a macroscopic sample of the same chemical composition. The special attention is focused on the analogy between the temperature range ΔT _m of melting of nanoparticles and the softening range for glasses.     

Electrochemical preparation of iron cuboid nanoparticles and their catalytic properties for nitrite reduction

Iron cuboid nanoparticles supported on glassy carbon (denoted nm-Fe/GC) were prepared by electrochemical deposition under cyclic voltammetric (CV) conditions. The structure and composition of the Fe nanomaterials were characterized by scanning electron microscopy (SEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). The results demonstrated that the Fe cuboid nanoparticles are dispersed discretely on GC substrate with an average size ca. 171 nm, and confirmed that the electrochemical synthesized nanocubes are single crystals of pure Fe. The catalytic properties of the Fe cuboid nanoparticles towards nitrite electroreduction were investigated, and enhanced electrocatalytic activity of the Fe nanocubes has been determined. In comparison with the data obtained on a bulk-Fe electrode, the onset potential of nitrite reduction on nm-Fe/GC is positively sifted by 100 mV, and the steady reduction current density is enhanced about 2.4-3.2 times.     

Size-dependent properties of magnetic iron oxide nanocrystals

The fine control of iron oxide nanocrystal sizes within the nanometre scale (diameters range from 2.5 to 14 nm) allows us to investigate accurately the size-dependence of their structural and magnetic properties. A study of the growth conditions of these nanocrystals obtained by thermal decomposition of an iron oleate precursor in high-boiling point solvents has been carried out. Both the type of solvent used and the ligand/precursor ratio have been systematically varied, and were found to be the key parameters to control the growth process. The lattice parameters of all the nanocrystals deduced from X-ray diffraction measurements are consistent with a structure of the type Fe3-xO4, i.e. intermediate between magnetite and maghemite, which evolves toward the maghemite structure for the smallest sizes (x=1/3). The evolution of the magnetic behavior with nanoparticle sizes emphasizes clearly the influence of the surface, especially on the saturation magnetization Ms and the magneto-crystalline anisotropy K. Dipolar interactions and thermal dependence have been also taken into account in the study on the nanoscale size-effect of magnetic properties.     

Polyaniline stabilized highly dispersed Pt nanoparticles: Preparation,characterization and catalytic properties

Platinum–polyaniline (2%Pt/PANI) composites are prepared using three precursor solutions of H₂PtCl₆ differing in the pH and thus in the type of dominating chloro–aqua or chloro–hydroxo complexes of Pt(IV). Various characterization techniques (X-ray diffraction, FTIR and X-ray photoelectron spectroscopy, and electron microscopy; TEM, HRTEM) were employed in order to elucidate the role of platinum precursor composition in the state of Pt-species formed in the Pt/PANI composites. In all three as-received Pt/PANI composites, platinum in two valence states Pt(IV) and Pt(II) was observed thus showing redox reaction under preparation of composites. The reduction of Pt(IV) is much easier in the precursor solution of alkaline pH, i.e. when the Pt(IV) complexes comprising aqua and/or hydroxo ligands are the dominating ones. Under these conditions the content of Pt(II)-species is ca. 7× higher than that of Pt(IV) ones. Reduction of as-prepared composites by NaBH₄ gives 2%Pt/PANI catalysts of well-dispersed Pt-metal particles of size ranging from 1 nm up to 4 nm. The average diameters (2.24 nm) of Pt-particles was slightly smaller in 2%Pt/PANI catalyst prepared using H₂PtCl₆ solution of acidic pH. Catalytic properties of Pt/PANI composites are tested in liquid phase hydrogenation of alkyne-type reactant, 2-butyne-1,4-diol (B3-D) and in gas-phase hydrodehalogenation of CCl₄. In the former, a classical example of alkyne to alkene and finally alkane hydrogenation, a catalytic behavior of Pt/PANI composites in terms of activity and selectivity was found to be similar to that observed for typical inorganic carrier supported Pt-catalysts. CCl₄ hydrodehalogenation on Pt/PANI leads initially to the formation of chloroform, which is the desired reaction product. However, in the course of reaction liberation of HCl brings about large changes, both in overall activity, as well as the selectivity, giving large amounts of dimeric C₂ products, especially C₂Cl₆. It is speculated that these changes follow from reprotonation of PANI which acquires Brönsted acidity used in dimerization of CCl₃ radicals.     

Size-dependent direct electrochemical detection of gold nanoparticles: application in magnetoimmunoassays

The effect of the AuNPs size, ranging from 5 nm to 80 nm, on the electrochemical response of screen-printed carbon electrodes (SPCEs) used as electrochemical transducers is investigated for the first time. A simple hydrodynamic modelling and calculation at the nanoscale level is applied so as to find the effect of the size of AuNP upon the electrochemical response. The results show that the best electrochemical response for AuNP suspension for the same concentration of total gold is obtained for the 20 nm sized nanoparticles. It is concluded that the Brownian motions avoid a better response for smaller AuNPs that should in fact be related with the best electrochemical signal due to their higher surface area. Finally, the size effect is studied for AuNPs acting as electroactive labels in an immunosensor that employs magnetic beads as platforms of the bioreactions. The best response for the 5 nm AuNPs in this case is due to the fact that in the immunosensing conditions the Brownian motions are minimized because the AuNPs contact with the electrotransducer surface is induced by the immunoreaction and the fast magnetic collection of the nanoparticles used as antibody labels upon application of a magnetic field.     

The thermodynamic aspect of melting and softening of nanoparticles

The specific features of the energy spectrum of stationary quantum states responsible for the melting and softening of a polyatomic system are investigated. It follows from the first principles of the quantum mechanics and statistical physics that the melting temperature of small nanoparticles can exceed the melting temperature of a macroscopic sample of the same chemical composition. The dependence of the temperature range of the transition of the polyatomic system to a microscopically labile state on the number of atoms is determined.     

Size-dependent catalytic activity of copper nanoclusters for oxygen electroreduction in alkaline solution

By using 2-mercapto-5-n-propylpyrimidine (MPP) as capping ligands, copper nanoclusters with different core sizes were prepared using a chemical reduction method. The as-prepared copper nanoclusters were loaded onto a glassy carbon electrode and their size effect on the electrocatalytic activity towards the oxygen reduction reaction (ORR) was investigated with electrochemical techniques in alkaline electrolyte. Cyclic voltammetric (CV) studies showed that the onset potential of ORR on smaller copper nanoclusters is more positive than that on larger copper nanoclusters. Compared to the larger clusters, higher current density of ORR was obtained using the smaller copper nanoclusters. These CV results indicate that the smaller Cu nanoclusters exhibit higher catalytic performance for ORR. In rotating-disk voltammetric studies, ORR on the synthesized MPP monolayer-protected copper nanoclusters is mainly dominated by a two-electron transfer pathway to produce H₂O₂.     

Pre-prepared platinum nanoparticles supported on SBA-15 – preparation, pretreatment conditions and catalytic properties

The primary objective of this paper is the complete characterization of Pt/SBA-15 catalysts prepared by sonication aided impregnation. During the experiments (i) the influence of sonication applied for introducing the already prepared platinum nanoparticles into the pores of the silica support, (ii) the pressure used for preparing the tablets and/or wafers for catalytic test, (iii) the temperature of heat treatment on the structural changes of catalyst samples, (iv) the effect of platinum particle concentration, and (v) the removal of rest organic matter from the catalysts by different procedures were systematically studied. The samples were characterized by nitrogen adsorption/desorption measurements (BET), TEM, SAXS, XRD and IR. The catalytic activity of the samples was also investigated in the reaction of cyclohexene hydrogenation.     

Monodisperse platinum nanoparticles of well-defined shape: synthesis, characterization, catalytic properties and future prospects

Monodisperse platinum nanoparticles with well-defined faceting have been synthesized by a modified polyol process with the addition of silver ions. Pt nanoparticles are encapsulated in mesoporous silica during in situ hydrothermal growth of the high surface area support. Removal of the surface regulating polymer, poly(vinylpyrrolidone), was achieved using thermal oxidation-reduction treatments. Catalysts were active for ethylene hydrogenation after polymer removal. Rates for ethylene hydrogenation decreased in accordance with the amount of Ag retained in the Pt nanoparticles after purification. Ag is most likely present on the Pt particle surface as small clusters. Future prospects for these catalysts for use in low temperature selective hydrogenation reactions are discussed.     

Preparation and catalytic properties of platinum dioxide nanoparticles: A comparison between conventional heating and microwave-assisted method

Using platinum chloride, sodium hydroxide, sodium acetate, and polyvinylpyrrolidone as starting materials, platinum dioxide nanoparticles with small particle diameter and narrow distribution were prepared by microwave irradiation and conventional heating, respectively. UV–vis spectrophotometer was used to trace the hydrolytic decomposition of platinum chloride and the formation of platinum dioxide nanoparticles. X-ray powder diffraction and transmission electron microscopy were employed to characterize the crystalline structure and the morphologies of the obtained nanoparticles. Gas chromatography was performed to investigate their catalytic properties for hydrogenation of cyclohexene. The results revealed that (i) the obtained nanoparticles have the same crystalline structure; (ii) the nanoparticles obtained by microwave irradiation were smaller and more narrowly distributed than those obtained by conventional heating; (iii) the activation time of the catalyst markedly influences the catalytic activity; (iv) the particles obtained by microwave irradiation showed higher catalytic activity than those obtained by conventional heating for hydrogenation of cyclohexene.     

Morphology-dependent redox and catalytic properties of CeO2 nanostructures: Nanowires, nanorods and nanoparticles

The redox features and the catalytic activities of ceria nanowires, nanorods and nanoparticles were comparatively studied. The morphology-dependent phenomenon is closely related to the nature of the exposed crystal planes. The CeO₂ nanoparticles mainly expose the stable {1 1 1} plane on the surface, whereas the rod-shaped nanostructures preferentially expose the reactive {1 1 0} and {1 0 0} planes, giving higher oxygen storage capacity and catalytic activity for CO oxidation. Although both the CeO₂ nanorods and the CeO₂ nanowires predominantly expose the reactive {1 1 0} and {1 0 0} planes, the CeO₂ nanowires favor to expose a large proportion of active planes on the surface, resulting in a much higher activity for CO oxidation than the nanorods.     

Synthesis and characterization of carbon-supported nanoparticles for catalytic applications

Metallic and carburized tungsten nanoparticles have been prepared using WCl₆ as precursor. The synthesis steps lead from tungsten hexachloride to tungsten carbide following the sequence WCl₆ → W⁰ → α-W₂C → WC. These different compounds have been mainly characterized by X-ray diffraction, TEM observations and adsorption measurements. The total reduction of tungsten hexachloride can be completed at T = 973 K. The carburizing of the metallic lattice into carbide WC is observed at 1223 K although incomplete. At this temperature, crystal growth phenomena appear which are not desired for catalysis applications. The synthesis of nanoparticles supported on activated carbon is accompanied by a decrease of the specific surface area of samples, especially during high temperature carburizing.     

Formation and catalytic activity of partially oxidized Pd nanoparticles

Combining multi molecular beam (MB) experiments and in-situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we have studied the formation and catalytic activity of Pd oxide species on a well-defined Fe₃O₄ supported Pd model catalyst. It was found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (~10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface. Their formation and reduction is fully reversible. As a consequence, the Pd interface oxide layer acts as an oxygen reservoir providing oxygen for catalytic surface reactions. In addition to the Pd interface oxide, the formation of surface oxides was also observed for temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature resulting in partially oxidized Pd particles between 500 and 600 K. It is shown that the catalytic activity of the model catalyst for CO oxidation decreases significantly with increasing surface oxide coverage independent of the composition of the reactants. We address this deactivation of the catalyst to the weak CO adsorption on Pd surface oxides, leading to a very low reaction probability.     

负载型Pt@Pd纳米粒子的制备与催化性能研究

用聚N-乙烯基乙酰胺接枝聚丙烯腈/聚苯乙烯(PNVA-g-PAN/PSt)聚合物微球为载体,H2PtCl6为金属源,无水乙醇为还原剂,将配位的Pt4+离子原位还原成为Pt纳米粒子负载于PNVA-g-PAN/PSt微球表面,进而在Pt纳米粒子上将Pd2+还原,制备了负载型Pt@Pd纳米粒子。用透射电子显微镜(TEM)对所制负载型Pt@Pd纳米粒子的形态与尺寸大小进行表征,结果显示,Pt@Pd纳米粒子均匀分布在聚合物微球表面;TEM和X-射线衍射结果显示,Pt@Pd纳米粒子的平均粒径随H2PtCl6与PdCl2比例的增加出现先增大后减小的变化趋势,粒径在9~22.2 nm之间;对肉桂酸产率的测定结果显示,该负载型Pt@Pd纳米粒子对碘代苯与丙烯酸交叉偶联反应有良好催化活性。     

Other catalytic properties

The behavior of fresh and aged EUROCAT V₂O₅–WO₃/TiO₂ SCR catalysts in the (I) oxidative dehydrogenation of light alkanes, and conversion of (ii) propan-2-ol, (iii) NO in the presence of propene and oxygen, (iv) propane and propene and (v) chloropropane is reported to analyse possible modifications of the catalyst properties during use and give a more comprehensive general picture of its surface and reactivity properties.     

Size distribution dynamics of fuel-borne catalytic ceria nanoparticles

Ceria-based fuel additives in diesel engines when dosed at above a certain concentration into the fuel have been shown to lead into bimodal exhaust particle size distributions in a previous study. In order to model this complex problem it is assumed that the soot aggregate size distribution (where each aggregate consists of individual primary particles) evolves fast toward a constant total surface area (determined by the “open” non-coalesced fractal aggregate morphology) and this surface represents a sink for the ceria nuclei. The latter undergo kinetic aggregation among them as well as are simultaneously scavenged by the soot particles. The mathematical model is formulated in terms of an aerosol population balance equation for the ceria particles. The governing parameter in the resulting dimensionless population balance equation is the ratio of the total surface area of the soot aerosol over the initial additive surface area. The latter is proportional to the total additive mass if the ceria critical nuclei are assumed to consist of one molecule, an appropriate assumption for high surface tension, metal oxides like ceria. The experimental results show that the critical additive concentration at the onset of the bimodal shape of the exhaust size distribution scales linearly with the exhaust soot mass fraction, hence the soot aggregates must have a quite “open” fractal structure in order for their total area to be proportional to their total mass. Although simple population balance models may provide some insight into the problem of interest, the experimental results show that models accounting for more complex interactions of additive and soot particles (potentially involving incomplete accommodation) must be investigated in the future.     

Synthesis and catalytic application of Pd nanoparticles in graphite oxide

Pd nanoparticles of 1–6 nm were synthesized in graphite oxide (GO) via cation exchange. The synthesis procedure involved immobilization of the precursor Pd(NH₃)₄(NO₃)₂ in GO, followed by reduction in flowing H₂. The resulting low-loaded Pd–GO material was characterized by X-ray diffraction (XRD), infrared (IR) spectroscopy and transmission electron microscopy (TEM). Structural characterization revealed that intercalation of the precursor took place in GO and the reduced Pd nanoparticles were situated both on the external surface and in the interlamellar space of the GO lamellae. The catalytic behaviour of Pd–GO was investigated in the liquid-phase hydrogenations of 3-hexyne and 4-octyne under standard conditions. For both reactants, marked turnover frequencies (18–36 s^?1) and pronounced (Z)-alkene stereoselectivities (93–98.4%) were obtained, indicating that Pd–GO was a highly active and stereoselective catalyst. For the stereoselective hydrogenation of 3-hexyne, Pd–GO exhibited an outstanding catalytic performance: at reactant:Pd (S:Pd) ratios ? 5000, complete conversions were achieved in 8–15 min and the (Z)-alkene stereoselectivities exceeded 98%.